Raman spectroscopy is a technique used to gain information about the chemical composition of a material and the state of that material. In this technique, the sample is illuminated with a bright light source and the wavelength distribution of the scattered light is measured. ‘Fingerprint’ chemical spectra are acquired when a laser excites different molecular vibrations. These molecular vibrations extract some energy from the excitation light resulting in scattered light with a longer wavelength. While information rich, this scattered light, which is the Raman signal, is very weak as only one out of about a billion laser photons excite molecular vibrations.
A conventional Raman system uses a spectrometer to determine the distribution of the scattered light. Because these spectrometers typically disperse colors of light in different directions and rely on free space propagation for their spectral separation and detection, they exhibit tradeoffs between spectral resolution, sensitivity, and device size. For example, one way of improving the detection sensitivity of a Raman signal involves increasing the size of the input slit to allow for more light to enter the system, which would in turn require a longer optical path-length for adequately separating dispersed colors without the loss of spectral resolution. This leads to a larger device and a detector array (e.g., a charge-coupled device (CCD)), increasing the cost and size of the Raman system. To get the highest performance, these detectors must also be cooled with thermoelectric refrigerators or liquid nitrogen, further increasing the cost and size of these systems.
The tradeoff among the size, cost, and sensitivity of Raman analyzers has resulted in two different instrument categories: (1) lower cost, handheld Raman analyzers with low sensitivity and (2) more expensive bench-top Raman analyzers with high sensitivity capable of measuring smaller concentrations of materials. (Here, sensitivity is defined as the rate of change in the output of the device/sensor to the rate of change of concentration. A device with a higher sensitivity can detect a weaker signal (lower analyte concentration) than a device with a lower sensitivity.) Handheld Raman analyzers are used mostly for identifying chemicals in high concentrations, for example, for explosives and illicit drugs. Bench-top Raman systems are used mainly for process monitoring in the pharmaceutical industry and as research tools in chemistry and life sciences laboratories. The sensitivity of these expensive bench-top systems is still much worse (orders of magnitude lower) than many other analytical techniques, such as mass spectroscopy.
Improving the sensitivity of Raman analyzers would unlock the benefits of Raman analysis in a whole host of new applications, including monitoring of low-concentration biochemical in pharmaceutical bioreactors, detection of highly potent street drugs especially synthetic opioids, monitoring of water and air contaminants, and monitoring of human physiology. Many of these applications require sensitivity levels that are outside the reach of today's Raman analyzers, so they are currently performed with more expensive, lower-throughput techniques, such as mass spectroscopy or chemical/enzyme-based assays.
There is also a desire or need to mitigate the performance tradeoffs of today's Raman analyzers. A more compact, cheaper Raman system that uses a lower power laser and lower cost detector would open up possibilities for using Raman spectroscopy in a wider range of settings, including the Internet of Things (IoT), as a clinical tool, and as an affordable consumer device.